The printing method in which the printing elements in the recording head in which the plurality of printing elements are provided are driven based on the multi-level image data so that an image is formed on the recording medium is known as the ink-jet printing method, the thermal printing method, etc.
The ink-jet printing method attracts the attention as the office use or others for the reasons that it enables high-speed printing, it can record the image having the gradation levels without requiring any special fixing processing to the plain paper, and the noise occurring at the time of printing is almost negligible.
Conventionally, various ink-jet printing methods have been proposed and the products have been produced and used practically.
In this ink-jet printing method, the ink-jet recording head in which the ink liquid chamber and the nozzle open to the ink liquid chamber are formed is used, and the pressure is applied to the ink in the ink liquid chamber according to image information, so that a small drop of the ink is discharged from the nozzle and made to adhere to the recording medium, such as the paper or the film, and the image is formed on the recording medium.
Moreover, the ink-jet recording head may be classified by the composition into the serial ink jet printer type and the line ink jet printer type.
In the serial ink jet printer, the ink-jet recording head forms the scanning line while scanning the paper in the width direction of the paper (main scanning), the paper is conveyed after one or more scans are completed, and the following scanning line is formed.
On the other hand, in the line serial printer, the nozzles are formed in the width direction of the paper to cover the whole width of the paper, and the printing is performed by conveying the paper without scanning it in the width direction.
The line ink jet printer is advantageous in that it forms one scanning line in the width direction at a time and the printing speed is high. However, there are the problems in that since the recording head itself becomes large, the whole printer must be large in size, and, when the printing with a high resolution is needed, the arrangement of the nozzles must be made to a high density and the manufacture cost of the ink-jet recording head becomes high.
In contrast, the serial ink-jet printer is advantageous in that the image is formed using the comparatively small recording head, and the device cost is low. For this reason, in recent years, many serial ink jet printers have been put in practical use.
The method of applying the pressure to the ink in the ink liquid chamber of the ink-jet recording head may be classified into the method using the piezoelectric element (PZT) which is the electro-mechanical transducer, and the method using the heat produced in the heating resistor like the thermal ink jet printer.
The ink-jet recording head is the integrated device which has the nozzle density in the range of 150-600 dpi and has the number of the nozzles in the order of several 100 nozzles. For this reason, it is difficult to produce all the ink-jet recording heads such that they are completely identical.
Moreover, there is also the variation in the materials used. For example, in the case of the ink-jet printing device using the PZT, the amount of displacement varies according to the material and process tolerance of the PZT. As a result, it is unavoidable that the variation arises in the discharging characteristics of the individual recording heads.
Conventionally, to obviate the above problem, the several methods have been proposed as disclosed in Japanese Laid-Open Patent Application Nos. 2000-085158, 2000-263818, 07-089100, 07-323551, 08-118727, and 11-020204.
The proposed methods of obviating the problem are mainly to change the voltage, the drive time, etc. of the drive waveform and adjust the ink drop speed and weight. However, if the drive waveform is changed, there is the problem that the power consumption in the PZT or the heating resistor may become large. Moreover, changing the drive waveform may make the dot configuration itself change, may make the satellite dot occur, or may make the dust occur, which will cause the image quality to deteriorate.
On the other hand, in the case of the thermal printing method, the recording head (the thermal recording head) in which the number of heating elements are formed is used, and any of the heating elements of the recording head is selected based on the image data and driven to generate heat, so that an image is formed on the thermal paper or the plain copy sheet.
The thermal printing device can record the image with the gradation levels by driving the heating elements of the recording head based on the multi-level image data and adjusting the heating energy, and does not need the post-processing, such as fixing, but has the features that the device size is small, the price is low, the noise at the time of printing is low, the operability is good, and the maintenance is easy to perform.
The recording head used for the thermal printing device may be classified into the line printing type using the recording head in which a large number of heating elements are arranged in a row according to the width of the recording medium and the printing is performed the thermal paper which is conveyed perpendicularly to the heating element row, and the serial printing type using the recording head in which a small number of heating elements are arranged in a row and the head is conveyed in the direction perpendicular to the moving direction of the thermal paper to perform the printing to the thermal paper.
Moreover, as for the thermal recording head, it is difficult to produce all the heating elements such that they are completely identical, and there is also the variation in the heat generating characteristics of the individual recording heads.